Arc Wander and Magnetic Fields

[Ray C]

In theory, agreed. In reality, calculate the magnitudes. Yes, a DC current in a wire produces a magnetic field -a very weak one which is why you need to wrap a wire thousands of times around a ferrous core before you get an effective electro magnet (right-hand rule: i.e: the mag fields are additive). The amount of magnetic field in an open-gap circuit? Something tells me if you calculated the relative strength of the electric vs magnetic fields under those circumstances, you'd see a negative double-digit number in the exponent. The context here was addressing another poster's comment that self induced mag fields were causing the plasma to wiggle. In this case, the electric field is driving the plasma insanely in one direction and the influence of the mag field is about as close to zero as you can get.

I'm going to disagree with you on this part (and this part alone). A current produces a magnetic field, period. There's nothing in Maxwell about current in a wire being different from current in an arc (plasma). It's right there in the equation that has curl B (B is the magnetic field), and J (J is the current density). http://en.wikipedia.org/wiki/Maxwell's_equations

 

[JohnAspinall]

I suspect we're 99% in agreement, and simply disagreeing about what comparison we're making.

I was not comparing electric field to magnetic field. I was comparing magnetic field from current in arc, to magnetic field from current in wire.
The magnetic field from 1 Amp in an arc is the same as the magnetic field from 1 Amp in a wire. (Assuming the geometry is the same, etc., etc.)

As for wiggle, we agree. The effect of magnetic field from arc current is tiny. The dominant factor is multiple equally-preferable arc paths leading to different parts of the workpiece.

 

[Ray C]

We're both wrong!

The wiggle depends on how many cups of coffee were consumed prior to arc welding and if a bee has suddenly decided to seek shelter from the bright lights and hang-out with you behind the welding mask. -Silly... How come we didn't think of that before?


Ray

I suspect we're 99% in agreement, and simply disagreeing about what comparison we're making.

I was not comparing electric field to magnetic field. I was comparing magnetic field from current in arc, to magnetic field from current in wire.
The magnetic field from 1 Amp in an arc is the same as the magnetic field from 1 Amp in a wire. (Assuming the geometry is the same, etc., etc.)

As for wiggle, we agree. The effect of magnetic field from arc current is tiny. The dominant factor is multiple equally-preferable arc paths leading to different parts of the workpiece.

 

[Shade]

EDIT (again): Someone will probably take exception to DC currents only producing electric fields. There are two distinct cases. 1) DC current traveling in a wire can indeed produce a magnetic filed. 2) We're not talking about the case of #1. We're talking about the case when an open gap has non alternating voltages at the terminals. This is a very different case and is effectively a capacitor where either air or the electron plasma itself is the dialectric. In this case, there is, can only be, an electric field. If you want to know the proof, let me know but, you will need several years knowledge of advanced algebra, calculus and matrix(vector calculus) theory. -Which are the fundamentals of Maxwell's equations.

I would love to see the proof. I have taken Multivariate and Vector Calculus, Differential Equations, Numerical Methods and 2 semesters of Quantum Mechanics, do I qualify. I also want to see how plasma is a di-electric.

 

[Tony Wells]

This is all well and good, but you cannot limit this effect to the fields around the arc itself. There are almost infinite variables involved in the position of the arc in relation to all the cabling and the material being welded. Whether there is a mag field around the arc or not, it is likely more the interplay of the fields generated by the current passing through the materials than the arc itself since they are a ferric conductor. Definitely agreeing with John.

And yes, I understand the math, etc, etc.

 

[Uglydog]

What's AWS?

No need for me to write about it. It was already written by Georg Ohm, Alessandro Volta, James Maxwell, Michael Faraday, Heinrich Hertz and the other contemporaries of the era who figured-out what electricity is. Written by me is nothing more than a history lesson -and now you know why those electrical terms are capitalized -because they are abbreviations for a proper noun.

Ray

AWS = American welding society.
I want to be careful to not offend any AWS writers and their ilk, as they put out a decent magazine and work hard to improve quality within the industry.
However, I wish they would spend a little more time on theory. Instead of merely application.

 

[Uglydog]

This is all well and good, but you cannot limit this effect to the fields around the arc itself. There are almost infinite variables involved in the position of the arc in relation to all the cabling and the material being welded. Whether there is a mag field around the arc or not, it is likely more the interplay of the fields generated by the current passing through the materials than the arc itself since they are a ferric conductor. Definitely agreeing with John.

And yes, I understand the math, etc, etc.

While all of this quit making any sense several posts ago, it has been very helpful.
It might be interesting to track how many views this theory stuff gets.

Wonder if once a month (week?) it would make sense for someone to systematically ask a strictly theory question which has obvious practical application for the masses and see if members respond with a goal of the gurus having fun gently poking each other, and occasionally making sense to the rest of us.

If it were me, I'd probably start with Machinery's Handbook as it's on most members shelf regardless of their penchant.

 

[Tony Wells]

Good idea, UD. I know a lot of it might bore some, and it's not alway applicable to hobby machining, but I'm sure there would be some interest. After all, everything that goes on in the shop has theory behind it, and is simply a manifestation of the theory, put to work. A discussion that allowed people to make the mental connections between theory and practice could be very valuable.

 

[Terrywerm]

Despite all of the great responses, I have to state my appreciation for sniggler's first response - the part about the current being annoyed. You just CAN'T make this stuff up, can you?? :roflmao:

 

[Ray C]

Well, it might actually be right... Magnets probably (almost certainly) cause some local polarity alignment in the steel and when the little electrons see an army of aligned atoms they stop and wonder for a while if they're courageous enough to pass the resulting field... Then again, the field could be pulling them in even though there's a closer escape path and they initially decided to go there instead. Quite a decision for the little fellows to make...

Here's a quick different "theoretical question"... On many occasions, I've had a glob of weld that always attracts the arc even though I've cleaned the daylights out of the surrounding area, cleaned the tip of the electrode and I'm holding the rod closer to the area I want to build-up. Now we all know this is the perfect setup to weld-in an inclusion but, for some things, you just don't care. Really wonder why this happens this way...



Oh, and BTW, on the problem of magnetic field strength from a couple posts ago, I'm assuming 25V OC and 100Amps. Once I calculate the strengths of the fields, I'll think-up a way to relate how much work each field is capable of.

Ray

Despite all of the great responses, I have to state my appreciation for sniggler's first response - the part about the current being annoyed. You just CAN'T make this stuff up, can you?? :roflmao: